Avoiding or deferring data copies

1. A computer-implemented method, comprising: using a virtual machine to perform: determining whether a set of program code comprises one or more references to a data object after an operation to generate a copy of the data object, wherein the set of program code is executable using the virtual machine; if the set of program code does not comprise one or more references to the data object after the operation to generate the copy of the data object, generating a set of optimized program code, comprising replacing the operation to generate the copy of the data object in the set of program code with an operation to update a reference in the set of optimized program code; determining whether the set of program code comprises an operation to generate a copy of a buffer object; and if the set of program code comprises the operation to generate the copy of the buffer object, generating a set of further optimized program code, wherein the set of further optimized program code comprises an allocation of one or more memory pages to store the buffer object with a copy-on-write parameter instead of the operation to generate the copy of the buffer object.

2. The method as recited in claim 1 , wherein the operation to update the reference comprises an operation to make an output object of a function reference the data object, wherein the data object is built by the function.

3. The method as recited in claim 1 , wherein the copy of the data object comprises an immutable object.

4. The method as recited in claim 1 , further comprising: executing the set of further optimized program code using the virtual machine, comprising: attempting to write to at least one of the memory pages storing the buffer object; and in response to the attempting to write, causing a copy of the at least one memory page to be generated.

5. A non-transitory, computer-readable storage medium storing program instructions computer-executable to perform: determining whether a set of program code comprises one or more references to a data object after an operation to generate a copy of the data object, wherein the set of program code is executable using a virtual machine; if the set of program code does not comprise one or more references to the data object after the operation to generate the copy of the data object, generating a set of optimized program code, comprising replacing the operation to generate the copy of the data object in the set of program code with an operation to update a reference in the set of optimized program code; determining whether the set of program code comprises an operation to generate a copy of a buffer object; and if the set of program code comprises the operation to generate the copy of the buffer object, generating a set of further optimized program code, wherein the set of further optimized program code comprises an allocation of one or more memory pages to store the buffer object with a copy-on-write parameter instead of the operation to generate the copy of the buffer object.

6. The non-transitory, computer-readable storage medium as recited in claim 5 , wherein the operation to update the reference comprises an operation to make an output object of a function reference the data object, wherein the data object is built by the function.

7. The non-transitory, computer-readable storage medium as recited in claim 5 , wherein the copy of the data object comprises an immutable object.

8. The non-transitory, computer-readable storage medium as recited in claim 5 , wherein the program instructions are further computer-executable to perform: executing the set of further optimized program code using the virtual machine, comprising: attempting to write to at least one of the memory pages storing the buffer object; and in response to the attempting to write, causing a copy of the at least one memory page to be generated.

9. The non-transitory, computer-readable storage medium as recited in claim 5 , wherein the set of optimized program code comprises a truncation or reallocation of memory storing the object.

10. A computer system, comprising: one or more hardware computing devices comprising one or more processors; and a virtual machine implemented by the one or more hardware computing devices, wherein the virtual machine is configured to: determining whether a set of program code comprises one or more references to a data object after an operation to generate a copy of the data object, wherein the set of program code is executable using the virtual machine; if the set of program code does not comprise one or more references to the data object after the operation to generate the copy of the data object, generating a set of optimized program code, comprising replacing the operation to generate the copy of the data object in the set of program code with an operation to update a reference in the set of optimized program code; determining whether the set of program code comprises an operation to generate a copy of a buffer object; and if the set of program code comprises the operation to generate the copy of the buffer object, generating a set of further optimized program code, wherein the set of further optimized program code comprises an allocation of one or more memory pages to store the buffer object with a copy-on-write parameter instead of the operation to generate the copy of the buffer object.

11. The computer system as recited in claim 10 , wherein the operation to update the reference comprises an operation to make an output object of a function reference the data object, wherein the data object is built by the function.

12. The computer system as recited in claim 10 , wherein the copy of the data object comprises an immutable object.

13. The computer system as recited in claim 10 , wherein the virtual machine is further configured to perform: executing the set of further optimized program code using the virtual machine, comprising: attempting to write to at least one of the memory pages storing the buffer object; and in response to the attempting to write, causing a copy of the at least one memory page to be generated.

14. The computer system as recited in claim 10 , wherein the set of optimized program code comprises a truncation or reallocation of memory storing the object.